Process for determining the resistivity of a formation through which a well equipped with a casing passes

ABSTRACT

Process for determining the resistivity (Rt) of a formation ( 9 ) surrounding a well ( 10 ) equipped with a casing ( 11 ) consisting of several casing segments ( 11.   i   , 11.   s ) with an overlapping part ( 1 ), and cement ( 3 ) in the overlapping part ( 1 ), 
         in which a current is injected into the casing ( 11 ) to cause a leakage current (Ifor) into a area ( 8 ) of the formation ( 9 ) offset from the overlapping part ( 1 ), the leakage current (Ifor) in the measurement area ( 8 ) is determined and is used to deduce the measured resistivity (Rm) of the formation,    a current is injected into the casing ( 11 ) to cause a current leakage (Icem) in the cement ( 3 ) of the overlapping part ( 1 ), the leakage current (Icem) in the cement ( 3 ) in the overlapping part ( 1 ) is determined, and is used to deduce the measured resistivity (Rcem) of the cement ( 3 ),    the measured resistivity (Rm) of the formation ( 9 ) is corrected using a factor to take account of the measured resistivity (Rcem) of the cement ( 3 ) to obtain the resistivity (Rt) of the formation ( 9 ). Application particularly to oil exploration.

TECHNICAL FIELD

This invention relates to a process for determining the resistivity of ageological formation through which a well equipped with an electricallyconducting casing passes.

The importance of resistivity diagraphs for oil exploration is nowuniversally accepted. It is known that the resistivity of a geologicalformation depends essentially on the fluid contained in it. This type ofresistivity measurement is frequently made on formations passed throughto decide whether or not to put an oil well into operation, so that itssaturation in fluid (mainly brine or fresh water, or hydrocarbonsincluding crude oil and gas, or a mixture of one or several of thesefluids). A formation containing brine has a much lower resistivity thana formation containing hydrocarbons.

STATE OF PRIOR ART

Measurements using prior art are made using systems provided withelectrodes to measure currents or voltages.

Resistivity diagraphs in open holes have been made for several decades.

We are now beginning to make resistivity measurements in wells reachinghydrocarbon reservoirs in operation and these wells are equipped with acasing. These measurements are used to determine the location ofwater-hydrocarbon interfaces and to monitor the change in level of theseinterfaces with time in order to monitor the behavior of the hydrocarbonreservoir and optimize its operation.

Resistivity measurements can also be made in wells in which nomeasurements were made before the casing was installed, particularly toincrease knowledge about the hydrocarbon reservoir, and possibly todetect production layers that were not identified initially.

These resistivity measurements in wells equipped with a casing are noteasy to make since the resistivity of the casing (of the order of 2×10⁻⁷Ω.m for a steel casing) is very small compared with the resistivity ofthe formation (between about 1 and 10³ Ω.m) and the casing forms abarrier to the transfer of current into the formation around the wellbeyond the casing.

The measurement principle consists of circulating an electric currentalong the casing under conditions such that a leak or current lossoccurs towards the formation. This leak depends on the resistivity ofthe formation, and is greater when the formation has a highconductivity. It can be evaluated by making measurements of the voltagedrop between electrodes placed at different levels in the well. Thesevoltage drops are of the order of a few nanovolts.

This type of measurement principle is described in a large number ofpatents and particularly in French patent applications FR-A1-2 793 031and FR-A1-2 793 032 issued by the applicant.

Refer to FIG. 1 that shows a section of a well 10 with centerline XX′,equipped with a metallic casing 11, in a formation 9. The level at whichthe measurement will be made is marked b. We consider a section a-c ofcasing 11 extending on each side of level b. A current is circulated inthe casing 11 with a return remote from level b, for example at thesurface. There is a current leakage Ifor in formation 9 and in terms ofthe electrical circuit, this current leakage is equivalent to thecurrent passing through a shunt resistance Rt located between level band infinity. The value of this shunt resistance Rt is representative ofthe resistivity (also called Rt) of the formation at level b. Thusaccording to Ohm's law, we can write:Rt=K(V _(b) ,∞/Ifor)

K is a geometric constant that may be determined particularly bycalibration, by moving into an impermeable area of the formation 9 inwhich the resistivity is already known through measurements made in theopen hole before the well is put into operation.

V_(b),∞ is the potential at level b with reference to a point atinfinity. It is measured using a measurement electrode eb placed atlevel b in electrical contact with the inside of casing 11 and areference electrode (not shown) that may be placed on the surface.

Ifor is the leakage current in the formation 9 at level b, and may forexample be determined using the method described in patent applicationFR-A1-2 793 031.

This method comprises three steps. In a first step, a current isinjected into the casing 11 at a point In1 at a longitudinal distancealong the formation 9 so as to cause a current leakage, and electrodesea, eb and ec placed at levels a, b and c respectively are used tomeasure the potential drops along casing sections a-b and b-crespectively. In a second step, a current is injected into the casing 11at point In2 at a longitudinal distance along the formation 9 located onthe side opposite the first point In1 to create a current leakage in theformation 9. Electrodes ea, eb and ec are used to measure the potentialdrops along casing sections a-b and b-c respectively. In a third step,the corresponding measurements in the previous two steps are combined inorder to obtain voltage values corresponding to a circuit formed by thecasing between the two injection points In1, In2 without any currentleakage to the formation. The leakage current Ifor in the formation 9 isdetermined from the first step or the second step, with the valuesresulting from the combination.

In one preferred variant, the first injection point In1 is located abovelevel a, the second injection point In2 is located below level c and thecombination is a subtraction of the measurements in the second step fromthe measurements in the first step.

There is another non-negligible barrier to sending current in theformation. This is the cement 3 that is poured into the well 10 to holdthe casing 11 in position. It fills in the inevitable space between theformation 9 and the casing 11. This cement 3 is equivalent to theresistance in series with the shunt resistance Rt due to the formation9.

The resistivity of cement can be known by laboratory measurements. Theresistivity of fresh cement is typically within a range between 1 and 10Ω.m. Once in place, the cement 3 is no longer directly accessible sinceit is behind the casing 11.

Furthermore, its resistivity varies firstly with time and secondly withthe medium in which it is located. Resistivity measurements in the wellequipped with a casing 10 may be made over several years or decadesafter the cement has been placed, and it is not known what has happenedto the cement during this time.

The porosity of the cement is of the order of 35% and when it is inplace, there is an ion exchange between the water contained in thecement and the water contained in the formation.

French patent FR-A1-2 807 167 application issued by the applicantdisclosed a process to determine the resistivity of a formation throughwhich the well equipped with a casing passes, taking account of theeffect of the cement. The authors found that the measured value of theresistivity of the formation could be corrected taking account of thethickness of the cement and its resistivity to obtain the “genuine”resistivity. “Genuine” resistivity means a value of the resistivity thatis as close as possible to the real value of the resistivity that isunknown and that is required.

Correction nomograms are used to find a correction factor for differentthicknesses of the cement layer, equal to the ratio between the requiredresistivity and the resistivity measured for the formation at themeasurement area, and this factor takes account of the ratio between themeasured resistivity of the formation and the resistivity of cement.

These nomograms are produced from mathematical models.

The thickness of the cement layer may be evaluated with acceptableprecision if the outside diameter of the casing 11 and the insidediameter of the well 10 before casing are known.

The resistivity of the cement is not known, and the estimate used tomake it is far from reality. The use of nomograms is not particularlyefficient for correcting the value of the measured resistivity of theformation to obtain the value of the resistivity of the formation. Theresult is an approximate value with mediocre precision.

DESCRIPTION OF THE INVENTION

This invention is intended to improve the precision of the correction byproposing a method to give a more precise value of the resistivity ofthe formation using nomograms, a value of the measured resistivity ofthe formation and a value of the resistivity of the cement obtained frommeasurements.

In order to achieve this, this invention consists of a process fordetermining the resistivity of a geological formation surrounding a wellequipped with a casing consisting of several casing segments followingeach other, in which two successive casing segments have an overlappingpart, and the cement located between the casing and the formation and inthe overlapping part between two adjacent casing segments,

-   -   in which a current is injected into the casing to cause a        leakage current into an area of the formation for which        measurements are required, offset from the overlapping part, the        leakage current in the measurement area is determined and is        used to deduce the measured resistivity of the formation in the        measurement area,    -   a current is injected into the casing to cause a current leakage        in the cement of the overlapping part, the leakage current in        the cement in the overlapping part is determined and is used to        deduce the measured resistivity of the cement,    -   the measured resistivity of the formation is corrected using a        factor to take account of the measured resistivity of the cement        to obtain the resistivity of the formation.

The correction factor is equal to the ratio between the resistivity andthe measured resistivity of the formation as a function of the ratiobetween the measured resistivity of the formation and the measuredresistivity of the cement, for a given cement thickness within the areabeing measured in the formation.

The correction factor may be given in nomograms starting from themeasured resistivity of the formation and the measured resistivity ofcement.

The measured resistivity of the formation can be deduced from theleakage current in the measurement area and the casing potential in themeasurement area with respect to a reference at infinity.

The measured resistivity of the cement can be deduced from the leakagecurrent in the cement in the overlapping part and the casing potentialin the overlapping part with respect to a reference at infinity.

The current injected into the casing to cause the leakage current in thecement in the overlapping part is such that it does not cause anyleakage current in the formation behind the overlapping part.

The leakage current in the measurement area and the leakage current inthe cement may be determined using a probe provided with measurementelectrodes in contact with the casing, this probe being moved in thewell to move to the measurement area and to the level of the overlappingpart, respectively.

Current may be injected into the casing using the probe that is equippedwith at least one current injector.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be better understood after reading the descriptionof example embodiments given for information only and that are in no wayrestrictive, with reference to the attached drawings on which:

FIG. 1, already described, shows the resistivity measurement principlein a well equipped with a casing

FIGS. 2A and 2B diagrammatically show a device for embodiment of theprocess according to the invention in two different measurementpositions;

FIG. 3 is an example of correction nomograms used to correct themeasured resistivity value of the formation;

FIGS. 4A to 4D show simulations of the variation of the leakage currentin the formation as a function of the depth, for various values ofresistivity of the cement, and for several resistivities of theformation, several diameters of casing segments, and several lengths ofthe overlapping part.

Identical elements on these figures are marked with the same referencemarks.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

We will now refer to FIGS. 2A and 2B that show an example of a devicefor implementation of the process according to the invention, in twodifferent positions. The device may be comparable to the devicedescribed in French patent application FR-A1-2 793 031 mentioned above.

The device shown comprises a probe 12 that can be displaced in a well 10formed in a geological formation 9 and equipped with an electricconducting casing 11.

The casing 11 is formed from several casing segments, usually made ofsteel. Only two of these casing segments have been referenced to avoidmaking the figures too complicated, one is called the upper segment 11.sand the other is called the lower segment 11.i. These casing segments donot all have the same diameter, and they are coaxial when they are inposition. These casing segments are lowered into the well 10 one afterthe other, from the thinnest to the thickest, and including anoverlapping part 1 between two casing segments 11.s and 11.i. The twosuccessive casing segments 11.s, 11.i do not have the same diameter,therefore there is a space 2 between the outside diameter of the lowercasing segment 11.i and the inside diameter of the upper casing segment11.s. The casing 11 is cemented to hold it in position in the well 10.The cement 3 is poured between the outside of the casing segments andthe formation in which the well 10 is bored. The cement 3 also entersthe space 2.

The probe 12 is connected by an electrical cable 13 to equipment 14located on the surface. This equipment 14 may comprise means ofacquisition and processing of data 15 supplied by the probe 12 andelectrical power supply means 16.

The probe 12 comprises a body 12.1 and a group of at least threemeasurement electrodes ea, eb and ec that can come into contact withcasing 11 by delimiting casing sections a-b, b-c. For example, thelength of these segments may be between 40 and 80 centimeters.Electrodes ea, eb, and ec may be installed at the ends of thearticulated arm 17 that connect them to body 12.1. These arms 17 areknown in themselves, they are extended to come into contact withelectrodes ea, eb and ec in contact with casing 11 when it is requiredto make measurements, and they are retracted when the measurements areterminated. When they are extended, these arms make good electricalcontact between the electrode fitted on them and the casing 11, and whenthey are retracted they can move the probe without friction inside thecasing 11. The probe 12 also contains two current injectors In1 and In2on each side of the group of measurement electrodes ea, eb, and ec.

Insulating connectors 18 are placed on each side around the body 12.1 ofthe probe 12 between the current injectors In1, In2 and the measurementelectrodes ea, eb, and ec to electrically isolate the measurementelectrodes ea, eb, and ec from the current injectors In1, In2. The spacebetween a current injector In1, In2 and the measurement electrode ea, ecclosest to it may be of the same order of magnitude as the space betweentwo successive measurement electrodes.

The current injectors In1, In2 may be made as described in the patentapplication FR-A1-2 739 031. They are also placed on the articulatedarms.

The device also comprises a current return electrode In3, remote fromthe injectors. It may be located on the surface at the same level as thecased wellhead 10 if the well is sufficiently deep, or it may be on thesurface but remote from the cased wellhead. The current injectors In1,In2 and the current return electrode In3 are powered with electricity,and they are connected to the electrical power supply means thatcomprise the above mentioned electrical power supply source 16 on thesurface and, depending on the case, a supplementary source (not shown)located in the probe 12 and appropriate switching circuits to changefrom one to the other.

On FIG. 2A, the probe 12 is in a position in which it can determine theresistivity of the formation that is located in a measurement area 8 atthe group of measurement electrodes ea, eb, ec. These measurementelectrodes ea, eb, ec are in contact with the lower casing segment 11.i,and they are remote from the overlapping part 1.

In this position of the probe 12, electrical current is circulated incasing 11 using at least one injector In1, In2 and the potentials aremeasured using measurement electrodes to determine the leakage currentIfor in the formation 9 in the measurement area 8, for example using theprocess described in the patent application FR-A1-2 739 031. Thisleakage current Ifor represents the resistivity of the formation in themeasurement area. All that is necessary to obtain the measuredresistivity Rm of the formation in the measurement area 8 is todetermine the casing potential 11 in the measurement area with respectto a reference at infinity. This can be done as described in patentapplication FR-A1-2 739 031 using electrode eb and a reference electrode(not shown), for example placed on the surface at a distance from thecurrent return electrode In3 or in the well.

The probe 12 is moved into a second position, the measurement electrodesea, eb, ec are then at the overlapping part 1.

In general, the measurement area 8 is located more deeply in theformation than the overlapping part 1. Obviously, it would be possibleto do the inverse.

The leakage current Icem in the cement 3 in space 2 is determined bycirculating current in the casing 11 using at least one current injectorof the probe 12 and measuring the potentials using the measurementelectrodes ea, eb and ec, this current representing the resistivity ofthe cement. As before, the measured resistivity Rcem of the cement 3 canthen be obtained by determining the potential of the casing 11 in theoverlapping part 1 with respect to a reference at infinity. The measuredresistivity of the cement Rcem can then be deduced from this value.

Correction nomograms like those illustrated in FIG. 3 and thosepresented in patent application FR-A1-2 807 167 can then be used, togive a value of the correction factor to be applied to the measuredresistivity Rm of the formation to obtain a precise value of therequired resistivity Rt. his factor is obtained starting from the cementthickness and the ratio between the measured resistivity Rm of theformation in the measurement area 8 and the measured resistivity of thecement Rcem.

In prior art, the resistivity of the cement was simply estimated and notmeasured, and there can be a large difference between the two values.Consequently, the correction was no longer valid for obtaining a valueof the required resistivity as close as possible to the real value.

The precision of the value of the resistivity of the formation obtainedwith this type of process is significantly improved.

On FIG. 3, the thickness of the cement 3 in the measurement area 8varies between 0 and 5 inches (about 12 centimeters).

This correction should be made when the ratio between the resistivity ofthe formation Rt and the resistivity of the cement Rcem is less than 1or when the cement layer is thick, more than about 1.5 inches (about 3.8centimeters); in other cases the effect of cement can be neglected.

A series of simulations was carried out to demonstrate the influence ofthe cement on the leakage current in the formation compared with otherparameters such as the resistivity of the formation, the diameters ofcasing segments, the length of the overlapping part. These simulationsare illustrated in FIGS. 4A to 4D. These simulations were made on amodeled well very similar to that in FIG. 2.

These simulations show the variations in the leakage current in theformation as a function of the depth between −8500 feet (about 2592meters) and −9100 feet (about 2775 meters).

A current of 1 A was injected into the lower casing segment 11.i, andthe resistivity of the cement 3 was varied between 0.5 and 20 Ω.m and 20Ω.m, the current electrode In3 being located at the surface of thewellhead 10.

In FIG. 4A, the length of the overlapping part is 200 feet (about 61meters), between depth −8500 feet (about 2592 meters) and −8700 feet(about 2653 meters). The lower casing segment 11.i has an outsidediameter of 4.5 inches (about 11 centimeters) and the upper casingsegment 11.s has an outside diameter of 7 inches (about 18 centimeters).The resistivity Rt of the formation is considered as being uniform andequal to 1 Ω.m. In the overlapping part 1 in which there is a lot ofcement, the current in the formation varies very significantly with theresistivity of the cement. These curves are considered as a reference.

The conditions in FIG. 4B are the same as in FIG. 4A except for theresistivity of the formation that was increased from 1 Ω.m to 10 Ω.m. Inthe overlapping part, the current is almost the same as the currentshown in FIG. 4A, which illustrates the small influence of theresistivity of the formation compared with the resistivity of thecement. Away from the overlapping part, the leakage current in theformation is lower since the resistivity of the formation has increased.

In FIG. 4C, the conditions are the same as in FIG. 4A except for theoutside diameters of the segments of the casing that have changed from4.5 to 7 inches, and from 7 inches to 9.625 inches (about 24centimeters) respectively. The thickness of cement between the twosegments of the casing 11.s, 11.i has not changed. The shape of thecurrent curves is approximately the same as in FIG. 4A but the currentamplitudes are slightly smaller. The dimensions of the casing segmentshave a small influence on the leakage current.

In FIG. 4D, the conditions are the same as in FIG. 4A except for thelength of the overlapping part that has increased from 200 feet to 500feet (about 152 meters) between −8200 feet (about 2500 meters) and −8700feet. The leakage current remains globally higher for shallower depths.It can be deduced that it will become easier to determine theresistivity of the cement as the length of the overlapping part isincreased. It can be seen that for an overlap of 500 feet, the curvescontain a plateau which makes the measurement almost independent of thedepth at which it was made.

These simulations show that the leakage current in the overlapping partis not very sensitive to the geometric configuration of the casing, andis much more sensitive to the resistivity of the cement. However, it isonly very slightly influenced by the resistivity of the formation.

1. Process for determining the resistivity (Rt) of a geologicalformation (9) surrounding a well (10) equipped with a casing (11)consisting of several casing segments (11.i, 11.s) following each other,in which two successive casing segments (11.i, 11.s) have an overlappingpart (1), the cement (3) located between the casing (11) and theformation (9) and in the overlapping part (1) between two adjacentcasing segments (11.i, 11.s), in which a current is injected into thecasing (11) to cause a leakage current (Ifor) into an area (8) of theformation (9) for which measurements are required, offset from theoverlapping part (1), the leakage current (Ifor) in the measurement area(8) is determined and is used to deduce the measured resistivity (Rm) ofthe formation in the measurement area (8), wherein a current is injectedinto the casing (11) to cause a current leakage (Icem) in the cement (3)of the overlapping part (1), the leakage current (Icem) in the cement(3) in the overlapping part (1) is determined, and is used to deduce themeasured resistivity (Rcem) of the cement (3), and the measuredresistivity (Rm) of the formation is corrected using a factor to takeaccount of the measured resistivity (Rcem) of the cement (3) to obtainthe resistivity (Rt) of the formation (9).
 2. Process according to claim1, wherein the correction factor for a given cement thickness (3) in themeasurement area (8) of the formation (9) is equal to the ratio betweenthe resistivity (Rt) and the measured resistivity (Rm) of the formation(9) as a function of the ratio between the measured resistivity (Rm) ofthe formation (9) and the measured resistivity (Rcem) of the cement (3).3. Process according to claim 1, wherein the correction factor is givenin nomograms starting from the measured resistivity (Rm) of theformation (9) and the measured resistivity (Rcem) of the cement (3). 4.Process according to claim 1, wherein the measured resistivity (Rm) ofthe formation is deduced from the leakage current (Ifor) in themeasurement area (8) and the potential of the casing (11) in themeasurement area (8) with respect to a reference at infinity.
 5. Processaccording to claim 1, wherein the measured resistivity (Rcem) of thecement can be deduced from the leakage current (Icem) in the cement (3)in the overlapping part (1) and the potential in the casing (11) in theoverlapping part (1) with respect to a reference at infinity.
 6. Processaccording claim 1, wherein the current injected into the casing (11) tocause the leakage current (Icem) in the cement (3) in the overlappingpart (1) is such that it does not cause any leakage current in theformation (9) behind the overlapping part (1).
 7. Process according toclaim 1, wherein the leakage current in the measurement area (8) and theleakage current in the cement (3) are determined using a probe (12)provided with measurement electrodes in contact with the casing (11),this probe (12) being moved in the well (10) to move to the measurementarea (8) and to the level of the overlapping part (1), respectively. 8.Process according to claim 7, wherein the current is injected into thecasing (11) using the probe (12) that is equipped with at least onecurrent injector (In1, In2).
 9. Process according to claim 2, whereinthe correction factor is given in nomograms starting from the measuredresistivity (Rm) of the formation (9) and the measured resistivity(Rcem) of the cement (3).
 10. Process according to claim 9, wherein themeasured resistivity (Rm) of the formation is deduced from the leakagecurrent (Ifor) in the measurement area (8) and the potential of thecasing (11) in the measurement area (8) with respect to a reference atinfinity.
 11. Process according to claim 10, wherein the measuredresistivity (Rcem) of the cement can be deduced from the leakage current(Icem) in the cement (3) in the overlapping part (1) and the potentialin the casing (11) in the overlapping part (1) with respect to areference at infinity.
 12. Process according claim 11, wherein thecurrent injected into the casing (11) to cause the leakage current(Icem) in the cement (3) in the overlapping part (1) is such that itdoes not cause any leakage current in the formation (9) behind theoverlapping part (1).
 13. Process according to claim 12, wherein theleakage current in the measurement area (8) and the leakage current inthe cement (3) are determined using a probe (12) provided withmeasurement electrodes in contact with the casing (11), this probe (12)being moved in the well (10) to move to the measurement area (8) and tothe level of the overlapping part (1), respectively.
 14. Processaccording to claim 13, wherein the current is injected into the casing(11) using the probe (12) that is equipped with at least one currentinjector (In1, In2).